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BIOL 111 Study Guide - Final Guide: Zygote, Planula, Sternum

Biology (Sci)
Course Code
BIOL 111
Suzanne Gray
Study Guide

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Phylum Cnidarian
Simplest of animals
Contain stinging cells called cnidocytes that assist in the capture of prey
and defense
Generally marine (i.e. jellyfish and corals)
General body plan consists of two layers of cells surrounding a central
cavity (radially symmetry)
Two General Forms:
1. Polyp: fairly sessile, in which the oral surface (mouth) is upward and the
aboral surface is attached to the substrate
2. Medusa: free swimming form in which the mouth is usually downward
**If organism has both forms in their life cycle, they are called dimorphic.
Generally, they have life cycles that alternate between a motile sexual medusa
stage and a sessile asexual polyp stage.
Phylum Cnidarian Class Hydrozoa Hydra:
Small polyp with no medusa stage
Hydra attaches its foot (basal disc) to the bottom of the dish and
extends its tentacles and body column
Move by gliding on its foot, inching along with its tentacles and foot, by
somersaulting from foot, to tentacles, to foot or by floating on a bubble
of gas secreted by the foot
Feeding Behavior:
Capture prey by using their tentacles and subdue them with toxic
stings of their nematocysts.
Food is transferred to their mouth, partially digested in the
gastrovascular cavity, and then absorbed by gastrodermal cells.
There intracellular digestion occurs. Undigested material is voided
through the mouth.
Do Hydra actively move to capture its prey or does it sit and wait for
the prey to bump into its tentacles? For the most part, they remain
attached to their substrate and let their tentacles drift with the water
currents. If a prey item (e.g., a zooplankton species) brushes into one of the
tentacles, the hydra quickly grabs onto it with the other tentacles subduing it
with poisonous threads/filaments from its nematocysts. Watch carefully to
see if this description fits the feeding behavior of your hydra.
How does radial symmetry of your hydra assist them with feeding?
Most cnidarians are opportunistic feeders and capture prey as they bump
into their tentacles, rather than stalking or chasing after its prey. Having
sensory and feeding structures (tentacles) radiating from their body
increases their abilities to sense and capture prey that may be coming at
them from any direction. This differs from animals that go after their prey. In
this case, being bilaterally symmetrical is more beneficial because the
sensory structures are concentrated in one area, which is moving in one
Why do we add acetic acid (vinegar) to the Hydra? The acid will induce
the Hydra to discharge their nematocysts from within their cnidocytes.
o Conical elevation with the mouth opening its tip
o Acts as the entrance (mouth) and exit (anus)
o Food catching arms that radiate from the hypostome
o Contain specialized stinging cells called cnidocytes used to
capture and defend
Gastrovascular cavity:
o Where digestion and absorption of food occurs
o Gas exchange (respiration) may also occur in the
gastrodermis (cells lining the cavity) as well as across the
Body Column:
o Composed of mesoglea (thin, gelatinous layer between the
epidermis and gastrodermis, visible as a thin dark line, acts as
a hydrostatic skeleton) sandwiched between two tissue layers
of cells:
Epidermis: outer layer of epithelial cells
Gastrodermis: inner layer of epithelial cells
Basal Disc:
o Foot
o Assists in locomotion
o Used to attach to the substrate
Cell Types:
Epitheliomuscular cells:
o Act as epithelial cells by covering and lining both surfaces
o Act as muscle cells by having contractile muscle fibers
o Longitudinal muscle fibers are located in the epidermis
o Circular muscle fibers are located in the gastrodermis (here
cells are able to engulf partially digested food particles
from the gastrovascular cavity and complete digestion
o Work to enable movement
What is the probable function of ciliated cells in the gastrodermis?
They, along with the muscular contractions of the cells in the gastrodermis,
create water currents, which circulate the food throughout the gastrovascular
cavity. This increases the distribution of nutrients to all the cells. Gas
exchange also occurs in the gastrodermis, so water currents help not only to
distribute food but also oxygen and to remove wastes.
o Located in the epidermis
o Most concentrated in the tentacles and hypostome
o Contains the nematocyst capsules (which are triggered by
cnidocils bristle-like projections on the outer surface of
the cnidocyte)
o Types of nematocysts: stinging, entangling and adhesive
o Nerve cells that extend along the base of the epidermis and
o Form a nerve net in each tissue layer
o Neurons running through the mesoglea connect the two
nerve nets together
o Sensory cells: type of neuron located in the epidermis
Sensitive to touch or chemicals in water
Impulses from sensory cells are relayed to neurons of
the nerve net that synapse with epitheliomuscular
cells results in coordinated muscular movement
Gland cells:
o Located in the epidermis of the basal disc
o Secrete a sticky substance used in anchoring the basal disc
to the substrate
o Also found in the gastrodermis, where they are used to
secrete digestive enzymes
Interstitial cells:
o Located between the epitheliomuscular cells of the
o Totipotent cells (act like stem cells by being able to divide
continuously and to differentiate into other cell types such as
neurons and cnidocytes
Sexual Reproduction: involves the union of a sperm and an egg cell
(gametes) which produce a zygote
o Sperm and egg are produced in the gonads (small
mounds in the epidermis of the body column)